1. Field of the Invention
The invention relates to a vehicular steering control apparatus and a vehicular steering control method More particularly, the invention relates to a vehicular steering control apparatus that has a steering transfer ratio variable device that changes the steering transfer ratio of the transfer from steering input device to steered wheels, and a power steering device that generates steering assist torque, and a vehicular steering control method.
2. Description of the Related Art
As one of steering control apparatuss of vehicles, such as motor vehicles, a steering control apparatus as described in, for example, Japanese Patent Application Publication No. JP-A-9-254804, is known which, in a vehicle having a power steering device that generates steering assist torque, detects the steering torque and the rotation angle of a steering shaft, and controls the assist torque so as to restrain changes in the steering reaction force attributed to a joint of steering shafts on the basis of the steering torque and the rotation angle of the steering shaft, and therefore restrains the deterioration of the steering feeling caused by fluctuations in the steering torque.
This steering control apparatus can restrain the changes in the steering reaction force attributed to changes in the torque transferred through the joint in association with rotation of the steering shaft, and therefore can restrain the deterioration of the steering feeling caused by fluctuations in the steering torque.
As a steering control apparatus for a vehicle, such as a motor vehicle or the like, a steering control apparatus is well known which is equipped with a steering transfer ratio variable device that changes the steering transfer ratio of the transfer from the steering wheel to the steered wheel by changing the relative rotation angle of steered wheel turning mechanism-side steering shaft with respect to a steering wheel-side steering shaft.
In the steering control apparatus equipped with the steering transfer ratio variable device, the change in the steering reaction force attributed to a change in the torque transferred through the joint is amplified by a change in the relative rotation angle of the steered wheel turning mechanism-side steering shaft with respect to the steering wheel-side steering shaft which is caused by the steering transfer ratio variable device, and at the same time, the phase relationship of the change in the steering reaction force to the rotation angle of the steering wheel is also thereby changed.
In the steering control apparatus of a vehicle that is not provided with a steering transfer ratio variable device as described above, the amplification of the change in the steering reaction force and the phase change thereof associated with the actuation of the steering transfer ratio variable device are not taken into account. Therefore, if the above-described steering transfer ratio variable device is applied to such a steering control apparatus, the change in the steering reaction force cannot be effectively restrained; moreover, the steering reaction force may increase or the steering reaction force may change unnaturally in some cases.
Furthermore, if the steering torque is detected and the control gain in the control of the assist torque on the basis of the detected steering torque is set high in order to restrain the aforementioned increase or unnatural change in the steering reaction force, the assist torque for the steering torque attributed to ordinary steering operation becomes excessive, resulting in deteriorated steering feeling.